This invention relates to a power conversion system which is used, for example, for an interruptable power supply.
In a conventional power conversion system comprising an inverter for forming an alternating current from a direct current, high frequencies are used for reduced size and improved performance.
A power conversion system in which a high-frequency voltage generated in an inverter is applied through a transformer and converted into a low-frequency voltage at a frequency changing circuit to reduce the size and improve the performance is disclosed, for example, in JP-A-6l-23637l and JP-A-6l-293l70.
FIG. 1 shows an example of a conventional power conversion system. In FIG. 1, reference numeral 1 designates a DC power supply, numeral 2 an inverter including gate turn-off thyristors (hereinafter referred to as GTO) 21, 22, 23, 24 and diodes 25, 26, 27, 28, numeral 3 a transformer, and numeral 4 a frequency changing circuit including a bilateral switching circuit 401 having GTO 41, 42 and a bilateral switching circuit 402 having GTO 43, 44. The switching circuits 401 and 402 include a center tap connection. Numerals 15 and 16 designate a reactor and a capacitor respectively of a filter 5 for improving the waveform of the output voltage of the frequency changing circuit 4, and numeral 7 a current detector. Numeral 100 designates a control circuit including control signal forming circuits 101 and 102 for forming a control signal applied to the GTO 21, 22, 23, 24 of the inverter 2 and the GTO 41, 42, 43, 44 of the frequency change circuit 4, a polarity signal forming circuit 103 for forming a polarity signal of an output voltage of the inverter 2, a voltage width determining circuit for determining the time width of the output voltage of the inverter 2, a short-circuiting prevention period generation circuit 104 for generating a short-circuiting prevention period signal for the inverter 2, a voltage polarity determining circuit 106 for determining the polarity of an output voltage of the frequency changing circuit 4, and a current polarity discrimination circuit for discriminating the polarity of the output current of the frequency changing circuit 4.
The operation of the above-mentioned conventional system will be described with reference to FIG. 2. The GTO 21, 22, 23, 24 of the inverter 2 are turned on and off in a manner to produce a voltage determined by the voltage width determining circuit 105, and short-circuiting prevention periods t.sub.1 to t.sub.2 and t.sub.4 to t.sub.5 are inserted to prevent the short-circuiting of the DC power supply 1. The frequency changing circuit 4, on the other hand, selects a switching element which is conducted in accordance with an output of the polarity signal forming circuit 103 for forming a polarity signal the output of the inverter 2 and the outputs of the voltage polarity determining circuit 106 and the current polarity discrimination circuit 107 for forming a polarity signal of the output voltage and output current of the frequency changing circuit 4 respectively thereby to convert a high-frequency voltage formed in the inverter 2 into a low-frequency voltage.
In the aforementioned prior art system, the starting time of the commutating operation for switching the switching element of the frequency changing circuit is set to coincide with the end time t.sub.2 of the short-circuiting prevention period of the inverter as shown in FIG. 2. During the short-circuiting prevention period t.sub.l to t.sub.2, however, a current path is formed from the transformer to the DC power supply, and therefore a voltage is produced at the output of the inverter, which in turn appears as an unnecessary reverse voltage at the output of the frequency changing circuit. As a result, the voltage which otherwise could be produced during the output period t.sub.2 to t.sub.4 is thus reduced during the commutation period t.sub.2 to t.sub.3 and the short-circuiting prevention period t.sub.1 to t.sub.2, thereby posing the problem of a reduced output voltage of the power conversion system.
Further, the commutation operation, though possible during the period when the polarity of the output voltage coincides with that of the current flowing in the reactor 5, cannot be effected and therefore an interrupting operation is required during the period when the polarity of the output voltage is different from that of the reactor current.